(1) Field of the Invention
This invention relates to an epoxy resin composition for the manufacture of a copper-clad laminate. More particularly, it provides the epoxy resin composition having an excellent adhesive property to metal.
(2) Description of the Prior Art
Due to a high density of a printed circuit board in recent years, it has been proposed to make the printed circuit board in a high multi-layered structure and the diameter of each through hole smaller, resulting in requirement for a printed circuit board material having a good drilling properties in time of drilling a through hole. Among the drilling properties, the occurence of smears is in serious problem in improvement of the drilling properties because it prevents conduction between an inner-layered circuit copper and a plated copper plated on the through hole causing a considerable decline of reliability for connection of the through hole. To solve such problem, there is a way to remove such smears.
Normally, makers of printed circuit boards have removed the smears by use of concentrated sulfuric acid, hydrofuluoric acid, chromic acid or the like, but these procedures have problems in view of safety. Further, such a conventional treatment provides cause to spoil the inner wall of the through hole and decrease reliability for connection of the through hole.
Further, as another means to prevent the decline of the drilling property due to the smears, there is a way to prevent occurrence of them. The smears are caused due to the resin, softened by a friction heat occurring at the time of drilling, being adhered to a section of the inner-layered circuit copper foil within the through hole. The conventional copper-clad laminate is manufactured by the use of the epoxy resin or the like as a prepreg resin which comprises dicyandiamide as a hardener. When such resins are hardened, the softening and melting-adhesion temperature thereof is low. Even though it has fully been hardened, its softening and melting-adhesion temperature is about 250.degree. C. It is generally said that the drilling temperature at drilling reaches about 300.degree. C., therefor, when drilling the conventional copper-clad laminate, the hardened resin is softened, and thus, occurrence of the smears is inevitable.
Still further, when the printed circuit board carrying parts is used, its use temperature may reach over 100.degree. C. Accordingly, it must have a long-term heat resistance in atmosphere. In the case that the laminate made of the conventional epoxy resins hardened by dicyandiamide is used for a long time in a drier at 170.degree. C., it takes about 500 hours until the ratio of maintaining flexural strength reaches not more than 50%. If occurrence of the smears will be prevented and the aforesaid time will be prolonged further, the copper-clad laminate will enhance reliability furthermore.
As a resin satisfying the aforesaid two requirements, there are epoxy resins hardened by a multi-functional phenolic compound. When applying for the printed circuit boards the copper-clad laminate of the epoxy resins hardened by the multi-functional phenolic compound, the occurrence of the smears at the drilling time is less than one-half of the conventional copper-clad laminate of the epoxy resins hardened by dicyandiamide.
In addition, the former is superior to the latter in view of long-term heat resistance. More particularly, the treatment time at 170.degree. C. of the former at the time when the ratio of maintaining flexural strength reaches not more than 50% becomes more than twice longer than that of the latter.
However, in view of adhesive property to metal such as copper foil, the epoxy resins hardened by the multi-functional phenolic compound are inferior to the conventional epoxy resins. For example, whereas the peeling resistance of the copper foil (its one side is roughened) having the thickness of 35 .mu.m of the conventional product is about 2 kg/cm, that of a product hardened by the multi-functional phenolic compound is 1 to 1.5 kg/cm.
Further, the peeling resistance of a product of which metal glossy surface is roughened and oxidized is also decreased. For example, concerning the adhesive property to a glossy surface of the copper foil provided with the above treatments, the peeling resistance of the conventional product is about 1.5 kg/cm, while that of the product made of the epoxy resins hardened by the multi-functional phenolic compound is about 0.5 to 1 kg/cm. Further, when measuring the peeling resistance of the test pieces of those products dipped in a hydrochloric acid, that of the dipped conventional product is nearly similar to that of a non-dipped conventional product, while that of a dipped product hardened by the multi-functional phenolic compound is reduced to half over that of a non-dipped product hardened by same.